deepCopy.js

function isObject(value) {
  return typeof value === 'object';
}

function deepCopy(value, hash = new Map()) {
  let newVal;

  if (hash.has(value)) {
    return hash.get(value);
  }

  if (Array.isArray(value)) {
    hash.set(value, newVal);

    newVal = value.map(item => {
      return deepCopy(item, hash);
    });
  } else if (value && isObject(value)) {
    newVal = {};

    Object.keys(value).forEach(key => {
      if (isObject(value[key])) {
        hash.set(value, newVal);
        newVal[key] = deepCopy(value[key], hash);
      } else {
        newVal[key] = value[key];
      }
    });
  } else {
    newVal = value;
  }

  return newVal;
}